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Author Topic: Peavey Bandit Novice repair  (Read 869 times)

phatt

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Re: Peavey Bandit Novice repair
« Reply #15 on: July 25, 2017, 04:50:13 AM »

That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #16 on: July 25, 2017, 01:18:09 PM »

That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.

thanks Phil I will get working on this stuff this week and report back
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #17 on: July 25, 2017, 01:28:45 PM »

So one more question that has been bothering me.  A theory on the damage to this amp I have was that since the original owner broke off the ground lug of the power cord and because of the damage to 1 of the 2 remaining lugs, I'm thinking that he plugged this thing in backwards (or upside down however you want to think of it).  Is this a reasonable deduction?
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DrGonz78

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Re: Peavey Bandit Novice repair
« Reply #18 on: July 25, 2017, 03:40:23 PM »

No. Reversing neutral and hot plugs on the amp will not make any REAL difference in this case. The safety ground being removed just means if there is leakage somewhere in the amp, to ground, the primary path will now only be the possibly grounded human guitar player. 

Stop trying to make a theory of what happened exactly. There are trained pros in the business of repair that can't EXACTLY tell you a WHICH transistor finally died when an amp suffered a major failure.

Edit: Just divide and conquer. Troubleshoot the amp by looking for obviously bad parts and replace. Test things (Transistors, Diodes, etc) in circuit with meter and see what might be bad. If a capacitor has started bulging then it is time to replace that guy.
« Last Edit: July 25, 2017, 03:44:55 PM by DrGonz78 »
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #19 on: July 25, 2017, 06:35:37 PM »

No. Reversing neutral and hot plugs on the amp will not make any REAL difference in this case. The safety ground being removed just means if there is leakage somewhere in the amp, to ground, the primary path will now only be the possibly grounded human guitar player. 

Stop trying to make a theory of what happened exactly. There are trained pros in the business of repair that can't EXACTLY tell you a WHICH transistor finally died when an amp suffered a major failure.

Edit: Just divide and conquer. Troubleshoot the amp by looking for obviously bad parts and replace. Test things (Transistors, Diodes, etc) in circuit with meter and see what might be bad. If a capacitor has started bulging then it is time to replace that guy.



Ok, great advice I will stop over thinking it and break out the multimeter. I'm stuck here at work all day wanting to be working on it!
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #20 on: August 04, 2017, 04:16:01 PM »

That suspect C71 cap is right next to the heatsink alloy which gets very hot and over time the excess heat has shrunk the plastic wrap and so it has split which is not a problem in itself,,, BUT Electro Caps don't fair well if exposed to a lot of heat and will dry out so yes it might be stuffed.
I would probably just replace it But not before I found that intermittent issue you just mentioned :o,, that sounds like it might be a cracked solder pad on one of the components.
Remember that Large components like Electro Caps are the obvious solder pad breaks but also components that run hot are also prime candidates for solder cracks. Big resistors that run hot often stress the solder joins as they heat up and cool down each cycle the solder eventually cracks.
Look for hair line cracks around the wire on the solder points. you need good eyes and good light.
HTH Phil.

I replaced c71.  Turned on amp and the cap blew on me.  I double checked thinking maybe I had installed backwards but I don't believe I did.  There is some bigger problem here.  I'm frustrated but hoping I can get some guidance on where to go next.  I don't see any other obvious visual signs of problems.  What kind of voltage should be present at the negative lead of c71?

Again thanks in advance for helping me to learn
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Enzo

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Re: Peavey Bandit Novice repair
« Reply #21 on: August 04, 2017, 06:43:15 PM »

Visual inspection is important, because if we see something burnt up, we need to deal with it.  But mostly, bad parts look like good parts.  What was the last time your car looked different if the water pump quit?  Or the battery went flat.  So we inspect with volt meters more than eyes.

Look at the layout, the + end of C71 faces the power transistor, the negative end closer to the end of the board.  The negative end of C71 connects to speaker hot basically.  The cap should not have more than 100v across it.  What are the power rails, 45v?  That means it ought never exceed 90v.  But caps hate reverse voltage, so if one of the main rectifiers shorts, it can put AC on that cap.
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #22 on: August 04, 2017, 09:43:31 PM »

Visual inspection is important, because if we see something burnt up, we need to deal with it.  But mostly, bad parts look like good parts.  What was the last time your car looked different if the water pump quit?  Or the battery went flat.  So we inspect with volt meters more than eyes.

Look at the layout, the + end of C71 faces the power transistor, the negative end closer to the end of the board.  The negative end of C71 connects to speaker hot basically.  The cap should not have more than 100v across it.  What are the power rails, 45v?  That means it ought never exceed 90v.  But caps hate reverse voltage, so if one of the main rectifiers shorts, it can put AC on that cap.

When I measure across the c71 connections (with cap removed) I get around -46v of Dc. When I switch to AC on the meter it bounces around all over the place.

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Enzo

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Re: Peavey Bandit Novice repair
« Reply #23 on: August 04, 2017, 10:46:41 PM »

Is music playing while you do?  This is a flying rail amp.
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #24 on: August 04, 2017, 10:51:18 PM »

No unfortunately nothing playing or plugged in. Speaker unhooked because of the voltage I was getting on the speaker wires.
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #25 on: August 05, 2017, 02:53:30 PM »

Is music playing while you do?  This is a flying rail amp.

I've been trying to understand this term "flying rail" and I ran across your post here. https://peavey.com/forum/viewtopic.php?t=42222

With that in mind I started to look at the transistors. I'm not really sure how to check them but they have continuity between all lugs but then I got to q11 and 1. I don't know where to check, are the lugs under the heat sink and curiously it is missing one of the nuts holding it down.
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MCM1910

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Re: Peavey Bandit Novice repair
« Reply #26 on: August 05, 2017, 03:16:10 PM »

I took out the board so I could see what q11 looked like and sure enough the board is burnt there as well as on one of the leads on q13...

The more I've looked at it the more I'm seeing signs that I'm not the first person to open this thing. Lots of scratching on the chassis around screws and bots that hold board in. The person I bought it from did not mention any of this.  Anyway I feel like I at least have a lead now.

I'll start looking up how to test transistors...
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